This invention relates to plastic enclosures for electrical apparatus, and more particularly to plastic enclosure boxes for load centers. Still more particularly, the invention relates to plastic boxes of the aforementioned type which may be used in an electrically grounded system.
The use of molded plastic enclosures for electrical apparatus in place of formed and fabricated sheet metal enclosures is increasing. Although the manufacturing cost for molded plastic enclosures is slightly greater than the manufacturing cost for highly tooled fabrication of a comparable sheet metal enclosure, the plastic enclosures offer several advantages over the sheet metal enclosures. For example, a plastic enclosure box can be made quicker and with fewer manufacturing steps (e.g. welding and painting), than a comparably formed and fabricated sheet metal box. The plastic box may be colored and is corrosion resistant without painting. Sharp edges and corners of the sheet metal enclosures, which offer potentially sharp cutting edges, are avoided in plastic enclosures by providing fillets and radii where appropriate. Moreover, the plastic enclosure is electrically insulative, thereby reducing electrical shock hazard of the enclosure and eliminating certain insulators required in metal boxes. Still another advantage is that many incidental features such as mounting, attachment and snap-together structure and the like can be readily incorporated in the molded plastic enclosure. Perhaps the most significant advantage afforded by the plastic enclosures is that they are lighter and stronger, i.e. more resistive to damage, than the sheet metal counterpart, and therefore lessen the shipping costs of the enclosures.
The foregoing advantages of molded plastic enclosures notwithstanding, electrical load centers provide certain unique considerations which have not been readily met by molded plastic technology. For example, load center enclosure boxes are provided in several lengths according to the number of circuits to be provided in the load center. Therefore, each load center having a different capacity would require its own molded enclosure box. Alternatively, a single enclosure box large enough to accommodate all sizes of interior panels and circuit capacities could be used, resulting in wasted space within the interior of the enclosure box for all but the largest capacity load center. Furthermore, a load center may be used as service entrance equipment, in which case the metal box becomes a part of an electrically grounded system, or it may be used as a sub-fed device (i.e. in a multiple unit dwelling) wherein the box is not required to be a part of the electrical grounding system. The particular end use of a given load center is not known at the time of its manufacture and perhaps not known at the time it is sold from distributor stock. Thus, where it is desired to use a plastic enclosure box in a grounded system application, it is necessary to provide special grounding attachment hardware on the conduit or metallic sheath cable which is connected to the box. While the molded load center box may incorporate features and advantages in sufficient numbers to justify extra cost, the various size cover-trims for such boxes may not justify the cost of plastic, and a metal cover-trims may be provided. The special grounding hardware should also accommodate grounding the cover-trims.